Here's a place where I can post my thoughts on new papers, provide updates on my projects, and post info that will eventually be on my website The Theropod Database - http://theropoddatabase.com/ . It will center on theropods, but may delve into other topics as well such as phylogenetics.

Friday, May 20, 2011

Is Saltopus a silesaurid?

Saltopus was recently redescribed by Benton and Walker (2011), who included it in Brusatte et al.'s (2010) archosaurian matrix. The result is that Saltopus is a dinosauriform more derived than Pseudolagosuchus but outside Silesauridae(including Lewisuchus)+Dinosauria. Since many recent analyses have synonymized Pseudolagosuchus and Lewisuchus, Tim Williams asked on the DML whether Saltopus would also be a silesaurid if Pseudolagosuchus and Lewisuchus were combined in Brusatte et al.'s matrix. Thanks to Tim, I have the Saltopus paper, so now the question can be answered.

First, I made a NEXUS file from Brusatte et al.'s matrix. Characters 52, 156, 159 and 170 had to have their states switched so that they could be properly ordered. More characters were ordered than Brusatte et al. had, with only 14, 51 and 96 left unordered. I then made a new OTU combining Lewisuchus' and Pseudolagosuchus' codings. Only two codings differed. Character 134 is the ventral emargination of the femoral head in anterolateral view seen in lagerpetonids. This was scored as present in Lewisuchus as well, and while Romer's (1972) figure does seem to show it, Arcucci (1998) determined this is actually a tibia. The other differing character is 148, the tibiofemoral ratio, which again is caused by the misidentified tibia. Thus the femoral codings of Lewisuchus were not taken into account for the combined OTU. The resulting tree is below-

Note that basically, the combined Lewisuchus+Pseudolagosuchus goes where Pseudolagosuchus used to. So it really doesn't answer our question. Also note that proper ordering changed a lot of relationships within Suchia from Brusatte et al.'s tree.

To resolve whether Saltopus would be a silesaurid in a topology where Lewisuchus is a member of the family, I constrained the results to agree with Nesbitt's tree for all taxa used in both analyses. The result is 47 steps longer and is interesting in-

- finding Gracilisuchus sister to Revueltosaurus+Aetosauria (as in some trees of Nesbitt's).

- finding Bromsgroveia to be at least as derived as Arizonasaurus but outside Lotosaurus+Shuvosauridae (as in Brusatte et al.).

- finding Scleromochlus to be a pterosauromorph (as in Brusatte et al.).

Saltopus forms a polytomy with Dinosauria and all four silesaurids, so may be a silesaurid or may not. And we're no closer to answering the question that when we began. It might be more useful to code it for Nesbitt's matrix.

Where are the pterosaurs recoded for Nesbitt's matrix, you may ask. I'm afraid once again work on a paper to be published has superceded this more recreational study. Luckily, it's work that coincides with my TWG analysis and should result in coauthorship of a new taxon. I will say that Nesbitt didn't code several pectoral and forelimb characters for pterosaurs, which when coded make them less likely to be archosaurs. However, the braincase of Pteranodon makes them more likely to be archosaurs. So after adding Preondactylus and Pteranodon, and recoding Eudimorphodon and Dimorphodon (but with no hindlimb characters looked at yet), they're still avemetatarsalians and take 17 steps to move them outside Archosauria by Peters' 'key taxa'.

Thanks. Something I didn't think to do is check Nesbitt's synaomorphy lists for Silesauridae and its subclades. Most are unknown in Saltopus due to incompleteness and poor preservation. "Proximal articular surface of the humerus continuous with the deltopectoral crest" is a silesaurid synapomorphy under ACCTRAN (known in Lewisuchus and Silesaurus), but seems more likely to be the primitive state for archosaurs. Interestingly, Saltopus is like most dinosaurs in having a humeral head which "seems to be quite separate from the deltopectoral crest." It is unlike Asilisaurus and Silesaurus in probably not having sacral ribs shared between centra. Besides these, no other characters can be determined.

Among dinosaurian characters of Nesbitt, Saltopus has a deltopectoral crest >30% down its humerus, a radius <80% of humeral length, the DELTRAN character of proximal articular surface of the humerus separated by a gap from the deltopectoral crest, and the ACCTRAN character of a forelimb–hindlimb length ratio less than 55%. It lacks the DELTRAN character of calcaneal tuber absent.

Based on that character distribution, Saltopus may be closer to dinosaurs than silesaurids.

Again you misunderstand "sister group". In that tree, Euparkeria is not the sister group to Proterochampsidae, it's the sister group to the clade containing Proterochampsidae, Crurotarsi and Avemetatarsalia.

Same goes for Lotosaurus and Yarasuchus. Yarasuchus is sister to the entire rest of the Poposauroidea. I could have drawn the tree with any poposauroid right below Yarasuchus without rearranging any branches.

In any case, all those areas you comment on are identical to Brusatte et al.'s original tree, so you can check his paper for the synapomorphies.